Quasi-molecular perspective on the planarity of 2D materials: the case of 2D MoS₂

The geometry of two-dimensional (2D) materials is explored, with a focus on monolayer MoS₂, through the lens of a quasi-molecular framework. In this approach, the extended 2D sheet is interpreted as a tiling of structural "tiles" - such as small molecular fragment units fully optimized at a high level of quantum chemical theory. This model provides a cost-effective and physically intuitive means of analyzing planarity and local bonding in large multi-layer systems. Specifically, we assess whether quasi-planar tiles can account for the observed planarity of 2D MoS₂, a material well-known for its S-Mo-S trilayer structure and trigonal prismatic coordination. By studying optimized Mo_nS_2n clusters (n = 1-12), we show that even though full planarity is not strictly preserved, a quasi-planar character emerges consistently. The results offer a compelling molecular perspective on extended materials and suggest that quasi-molecular tiling can effectively capture the local geometry and electronic structure of 2D MoS₂.